[tip:perf/core] perf: Update userspace page info for software event

[tip-bot for Shaohua Li]

Commit-ID:  6a694a607a97d58c042fb7fbd60ef1caea26950c
Gitweb:     http://git.kernel.org/tip/6a694a607a97d58c042fb7fbd60ef1caea26950c
Author:     Shaohua Li <shli@xxxxxx>
AuthorDate: Thu, 5 Feb 2015 15:55:32 -0800
Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Wed, 18 Feb 2015 17:01:45 +0100

perf: Update userspace page info for software event

For hardware events, the userspace page of the event gets updated in
context switches, so if we read the timestamp in the page, we get
fresh info.

For software events, this is missing currently. This patch makes the
behavior consistent.

With this patch, we can implement clock_gettime(THREAD_CPUTIME) with
PERF_COUNT_SW_DUMMY in userspace as suggested by Andy and Peter. Code
like this:

  if (pc->cap_user_time) {
	do {
		seq = pc->lock;
		barrier();

		running = pc->time_running;
		cyc = rdtsc();
		time_mult = pc->time_mult;
		time_shift = pc->time_shift;
		time_offset = pc->time_offset;

		barrier();
	} while (pc->lock != seq);

	quot = (cyc >> time_shift);
	rem = cyc & ((1 << time_shift) - 1);
	delta = time_offset + quot * time_mult +
		((rem * time_mult) >> time_shift);

	running += delta;
	return running;
  }

I tried it on a busy system, the userspace page updating doesn't
have noticeable overhead.

Signed-off-by: Shaohua Li <shli@xxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
Cc: Frederic Weisbecker <fweisbec@xxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/aa2dd2e4f1e9f2225758be5ba00f14d6909a8ce1.1423180257.git.shli@xxxxxx
[ Improved the changelog. ]
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 kernel/events/core.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index e580e0f..fef45b4 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -6123,6 +6123,7 @@ static int perf_swevent_add(struct perf_event *event, int flags)
 	}
 
 	hlist_add_head_rcu(&event->hlist_entry, head);
+	perf_event_update_userpage(event);
 
 	return 0;
 }
@@ -6592,6 +6593,7 @@ static int cpu_clock_event_add(struct perf_event *event, int flags)
 {
 	if (flags & PERF_EF_START)
 		cpu_clock_event_start(event, flags);
+	perf_event_update_userpage(event);
 
 	return 0;
 }
@@ -6666,6 +6668,7 @@ static int task_clock_event_add(struct perf_event *event, int flags)
 {
 	if (flags & PERF_EF_START)
 		task_clock_event_start(event, flags);
+	perf_event_update_userpage(event);
 
 	return 0;
 }
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